I mounted my windows2000 partition (which is on another drive) by:
mount -t vfat /dev/hdb1 /mnt/vfat

Since my Linux(RH9) partition doesn't have enough space, I wanted to put some of my files in the windows partition. So I tried changing the permissions, so that I can RW it in my non-root login, using chmod:

# chmod 775 My\ Documents/
chmod: changing permissions of `My Documents/' (requested: 0775, actual: 0755):
Operation not permitted

I tried looking at man mount, to see if there are any mount options that I should have used. But by default mount is rw.

Can anyone help me know what/why the problem is.

Thanks.

The only thing I can think of is that the file system is mounted Read-Only.
Try adding -o rw to your mount command.

Also, changing permissions on a vfat file system does not work in the same way as in Linux.

when you add the mount point to your fstab you have to specify in the options that you want regular users to have r/w access to the drive instead of ro...
adding something like umask=000 or uid=500 (if your userid is 500) will work.